A great deal of public attention has recently been focused on the problem of heavy metal contamination of drinking water supplies; the ingestion of metals such as mercury, even in very small quantities, is known to cause acute and chronic illness in both humans and other animals. In response to this inherent danger, a number of states and the Federal government have enacted legislation to control further pollution of surface and ground-water supplies. Strict requirements have been imposed on industry to insure that heavy metals are removed prior to effluent discharge. The procedures required to accomplish this, such as carbon absorption, flocculation-sedimentation, and treatment with ion-exchange resins, may prove to be very costly. Thus, it would be desirable to develop a technique for removal of metals from water which is rapid, efficient, and inexpensive, and at the same time requires a low level of technology. Another direction in which this technology may be applied is for the removal of valuable metals from mine drainage.
One direction in which there has been a heavy concentration of research is the use of microorganisms for leaching metals (and other substances) from water. Bacteria have long been used for leaching a variety of metals from various sources. For example, U.S. Pat. No. 3,937,520 describes the use of anaerobic bacterial of the genera Ferrobacillus and Thiobacillus for in situ "mining" of minerals; in this case, the bacteria produce sulfuric acid which leaches the mineral from the stratum. U.S. Pat. No. 3,923,597 discloses the use of a genetically engineered Pseudomonas which binds mercury by the production of a mercury-binding protein. It is also known to utilize certain phototropic bacteria to remove silver from photographic processing effluents (U.S. Pat. No. 4,135,976).
It is also possible to extract metals by employing microorganisms other than bacteria. For example, U.S. Pat. No. 4,293,333 discloses the use of funji to remove precious metals from dilute aqueous solutions. Also known to be particularly useful are various species of blue-green algae and green algae. U.S. Pat. No. 3,725,291 describes a sorbent which utilizes plant material, including algae, to adsorb metals such as mercury, uranium, silver, platinum, palladium and gold from aqueous solutions. However, this system has at least two drawbacks: it recommends the use of pulverized algal cells; and also requires that the plant adsorbent be combined with a polymeric binder. Other methods are known which utilize whole algal cells without the necessity for combination with a binder. For example, Nakajima et al., (Eur J. Appl. Microbiol Biotechnol (1982) 16:88-91) teach the use of whole Streptomyces or Chlorella cells immobilized in a polyacrylamide gel for uranium extraction. Sloan et al., (Proc. Ind. Waste Conf. (1984), Sec. 9, Metal Works, 423-429) further describe the use of the algae Nostoc, Anabaena and Chlorella for copper, cadmium, lead and zinc extraction.
Conspicuously absent among the known uses for microorganisms in metal recovery is a technique by which a selective extraction of precious metals such as gold and silver, can be accomplished. A process which could selectively recover these metals from old mining dumps, mineral leaching operations, and industrial processes which use these metals would clearly have tremendous commercial potential. Although it has been known to use microbial systems to extract metals from waste systems, very frequently particularly desirable metals, such as gold and silver will be found in solution with other metals which are not of interest, but which will bind to the microbial cells as well as the precious metals. Therefore, what is truly needed is a method by which the microbial cells' ability to bind metals can be manipulated in such a manner that the particular metal or substance of interest will either be bound to the substantial exclusion of unwanted metals, or a method for selectively recovering the precious metals from microbial cells to which they are bound without also recovering other undesirable bound metals.
It has now been unexpectedly discovered that certain microorganisms under controlled conditions of pH and salt concentration can be used to selectively bind gold, silver or platinum, while essentially preventing the binding or causing the release of a number of competing metals. The present invention, which may employ either dead or living cells, without the necessity for using costly ion-exchange resins, provides a simple, inexpensive, low-technology method of extracting valuable metals from even dilute aqueous solutions. This method also provides a means for mercury extraction. At the same time, the present method provides a simple procedure for eluting metals other than gold, silver, mercury and platinum from microbial cells to which they are bound. It has also been found that the metals gold, silver and mercury can be selectively recovered from microbial cells to which they are bound, even if other unwanted metals are also bound to the microbial cells, regardless of the pH at which binding occurs. Also provided is a method of extracting certain oxoanions from waste waters containing them, and novel compositions useful in metal extraction.